New Fossil Is Oldest Upright Walker

by Clara Moskowitz | March 19, 2008 08:00pm ET

A 6 million-year- old thighbone, or femur (center), represents an upright walker of one of the earliest human ancestors (Orrorin), and resembles 2-3 million year old thighbones of australopiths (left, bottom). Thighbones of Homo (right) mark a transition toward a more modern gait about 2 million years ago.Credit: artwork and composite byJohn Gurche, photograph by Brian Richmond

A 6 million-year-old early relative of modern humans
apparently walked on two feet, pushing back the origins of so-called
bipedalism, according to a new study of a fossil found in Kenya.

"I would say at this point it’s the earliest fossil
hominin that we can clearly identify as bipedal,"
said paleoanthropologist William Jungers of Stony
Brook University,
who conducted a quantitative analysis with Brian Richmond of George Washington
University of a fossilized
femur bone from the species named Orrorin
tugenensis. It is one of the earliest known pre-humans.

The researchers compared the shape of this thigh bone to
those of modern humans, apes and other early hominins, including Australopithecus (the species to which
the famous "Lucy"
fossil belongs). The team determined that the femur bears the signatures of
bipedalism, or walking upright on two feet.

The research, funded by the National Science Foundation and
Stony Brook and George Washington universities, is detailed in the March 21
issue of the journal Science.

Carol Ward, an anatomist at the University of
Missouri-Columbia who was not involved in the research, said the team's
findings are significant.

"No detailed study had ever been done on this fossil,
and they did a very solid comparative metric analysis," she said.

Debate about
tree-climbing

What's special about O.
tugenensis, and other early humans that lived between 6 million and 2
million years ago, is that they not only travelled on the ground on two legs
but also retained the ability to climb trees, Jungers said.

"These are bipedal walkers that were also using the
tress for food, sleeping and escaping from predators," Jungers told LiveScience. The researchers think O. tugenensis was a climber because of a
finger bone also found belonging to the species. The finger is curved, Jungers
said, a sign that it was used to grasp trees.

Ward said she isn't convinced this species or its later
relatives spent a lot of time in trees.

"Everyone agrees they were well-adapted to walking
upright on the ground," she said. "People differ on how important
tree climbing was. I think we can't say yet. We need more fossils."

Eventually, the ancestors of modern humans completely lost
their expert climbing abilities.

"What happens at about 2 million years ago is really
fascinating, because you relinquish this very successful body plan, and what
emerges is a body plan that's much more similar to yours and mine," Jungers
said.

At this point, our ancestors gave up their curved finger
bones and gained longer hind legs, perfect for walking long distances and
running but not as well suited to scrambling around trees.

The O. tugenensis
fossils were discovered in 2000 by a team led by French researchers Martin
Pickford and Brigitte Senut. The find was dubbed "Millennium Man."

Pickford and Senut were the first to propose that the
species was bipedal, but it wasn't until Jungers' and Richmond's new study that this could be
confirmed.

Clarifying the
hominin ancestry

The fossils' discoverers had suggested that Orrorin was a direct ancestor of modern
humans, with special similarities to us. Jungers and Richmond found that these ancient fossils
actually have much more in common with Australopithecus,
an extinct early hominin made famous by the discovery of "Lucy." Australopithecus appeared about 4
million years ago, 2 million years after O.
tugenensis.

Ward agreed that the new study disproved the hypothesis that
Orrorin was a direct modern human
ancestor.

"This certainly puts the nail in the coffin on that
idea," she said. "They’ve very carefully demonstrated that it looks
like Australopithecus."

Both Australopithecus
and O. tugenensis were smaller than
modern humans and stocky, Jungers said. They had big teeth, projecting faces
and small brains, closer to the size of chimpanzee brains than ours.

Though O. tugenensis
was not our direct ancestor, it was part of the group of early hominins that
eventually gave rise to our genus Homo,
as opposed to the related group from which chimpanzees emerged. The study of
the Orrorin fossils helps scientists
narrow down when humans and
chimpanzees split.

"This clearly post-dates that split, so it gives us a
minimum date of six million years ago for humans splitting off," Jungers
said.

Clara Moskowitz

Clara has a bachelor's degree in astronomy and physics from Wesleyan University, and a graduate certificate in science writing from the University of California, Santa Cruz. She has written for both Space.com and Live Science.